This invention relates to an induction motor drive operable in the field weakening mode (constant horsepower operation) and more particularly to a motor drive using a vector control operable in the field weakening mode using full inverter output voltage.
Vector control of an induction machine allows the optimal use of the dynamic capabilities of the motor as a prime mover. Vector control relies on the elimination of the undesirable dynamics of the stator impedances, which can vary by more than 100%, by using stator current control. The stator currents are measured and used in a closed loop control which provides as an output the required voltage command, which is supplied to the motor.
The vector control can be explained by noting that generically AC and DC machines are analogs, and this analogy becomes clearer if the stator currents of an AC machine are converted from a stationary reference frame to a synchronized rotating reference frame. In a separately-excited dc shunt machine, torque is produced as a product of field flux produced by field current, and armature current. Because the field flux and armature current are physically located so as to be orthogonal, there is no interaction or coupling between the torque producing factors (not taking into account saturation and armature reaction effects). No coupling between the armature current and flux means that if the armature current, for example, is varied, the torque will change proportionally without variation in flux or field current producing the flux.
In an AC machine the torque is also proportional to the product of flux and current. The flux and current are both alternating variables and it is necessary to resolve them into two quantities using the equivalent two phase machine model with an x-y reference frame, which rotates together with the rotor flux. Stator current components i.sub.1x, i.sub.1y influence both the rotor flux and torque. That is, if either component of the current is changed to influence the torque, the flux will also change. However, a "decoupling" can be accomplished by orienting the rotating x-y coordinates to a position where the y axis coincides with the resultant rotor flux vector. In this new reference frame, only the stator component i.sub.1y influences the flux, while the i.sub.1x component builds the torque. Thus by using a torque producing current command of i.sub.1x * proportional to slip and a flux producing current command of i.sub.1y *, determined from machine voltage, rapid torque response and accurate torque regulation can be achieved, since the two commands are decoupled. Both current component commands are dc quantities during steady state operation.
When constant horsepower operation is desired in a vector control, several approaches have been followed in the past. The commanded flux level can be open loop controlled by varying the field producing current command i.sub.1x *, in a way that always assures that enough voltage reserve is left for current control. The open loop-field oriented control (flux determined open loop), allows the regulation of the rotor flux only when the commanded current and the actual stator current coincides. This requires that the bandwidth of the current control is wide enough to guarantee that, even at the maximum stator frequency, the current error (amplitude and phase) is acceptable. At the same time, the rating of the motor and inverter have to be matched so that enough voltage reserve is always available for the current control to work. This control requires knowledge of dc link voltage available and full six step inverter operation cannot be achieved.
Another control method of obtaining constant horsepower is to transition to a voltage control mode of operation, by computing the required switching pattern inversely proportional to motor speed. This control requires a knowledge of the dc link voltage available and full six step inverter operation cannot be achieved.
An angle control with the angle between the main flux and the total stator current being controlled, can be utilized to obtain full six step inverter voltage operation above base speed (which includes constant horsepower region) if the torque producing component of the stator current is used to generate the angle command. However the angle control does not achieve the dynamic response of the vector control system.
A six step inverter can be used where the magnitude of the dc link voltage is controlled. Full six step operation in the constant horsepower region can be achieved, however it is suitable only for low performance drives such as compressors and pumps and is not suitable for servo and spindle drives, since precise speed control with smooth changes in speed cannot be obtained.
It is an object of the present invention to provide an induction machine drive operable in the constant horsepower region using full inverter voltage and having good dynamic response.
It is a further object of the present invention to provide an induction machine drive operable in the constant horsepower region using full inverter voltage without sacrificing any of the high performance capability in the constant torque low speed mode of operation.
It is a still further object of the present invention to provide an induction machine drive operable in the constant horsepower region using full inverter voltage while improving the performance capability in the constant torque low speed mode of operation.